The present invention relates to a semiconductor memory device, and more specifically to a semiconductor memory device backuped by a battery.
The semiconductor memory device can be roughly divided into read-only memory (ROM) device and random-access memory (RAM) device. Since almost all RAM devices are volatile, data stored in the memory cells are dissipated whenever voltage supply fails.
In various OA (office automation) apparatus widely used recently, many RAM devices are incorporated therein as semiconductor memory devices, data stored in the memory cells are usually held by a battery provided for the apparatus, whenever the ordinary power supply fails. FIG. 5 shows an example of prior-art system backed up by a battery.
Under the normal power supply conditions, power is supplied from a DC power source 51 to a semiconductor memory device 52. However, when power supplied from the DC power source 51 fails, power is supplied from a battery back-up circuit 57 to the device 52. Usually, a power switching circuit 56 and a battery back-up circuit 57 are connected in series between the DC power source 51 and the semiconductor memory device 52.
In more detail, a voltage of 5V is ordinarily outputted from a power supply terminal OUT of the DC power source 51. Therefore, current i flows through a Zener diode D1 and a resistor R53, and a voltage across the resistor R53 causes current to flow via a resistor R52 as a base current of a bipolar transistor 54. Therefore, a voltage across the resistor R52 turns on the bipolar transistor 58, so that an output of the DC power source 51 is supplied between a power supply terminal V.sub.CC2 of the semiconductor memory device 52 and a ground terminal V.sub.SS. Here, since a rechargeable battery (i.e. Ni-Cd cell) is used as a battery E of the battery back-up circuit 57, the battery is kept charged via a resistor R55.
However, when the output of the DC power source 51 begins to drop from a time point t1 as shown in FIG. 6 and therefore the voltage between the power supply terminal OUT and the ground terminal GND drops below a breakdown voltage of the Zener diode D1, since the Zener diode D1 is turned off, the two bipolar transistors 54 and 58 are both turned off, so that electric charge accumulated in a capacitor C3 is discharged via a resistor R55. In this case, voltage V.sub.N1 of node N1 drops relative to a voltage V.sub.E of the battery E. Therefore, a forward bias voltage is applied to a diode D2 at a time point t2 at which the voltage V.sub.N1 drops by a value corresponding to a junction potential of the diode D2, relative to the battery voltage V.sub.E. That is, at this time point t2, power supply starts from the battery backup circuit 57 to the semiconductor memory device 52, instead of the DC power source 51.
As described above, in the prior-art device, power supplied to the semiconductor memory device 52 is switched by the power switching circuit 56 from the DC power source 51 to the battery back-up circuit 57 or vice versa.
In the prior-art device, however, since the power switching circuit 56 is additionally required, there exists a problem in that the packaging efficiency of the semiconductor memory devices 52 is reduced. In addition, when the device is backed up, since the semiconductor memory device 52 must be so activated as to hold data, another memory device control circuit must be integrated, thus further decreasing the packaging efficiency of the memory device.